1. Field of the Invention
The present invention relates to a proximity fuse, and more particularly pertains to a laser radar-proximity fuse with a laser-range measuring device and mask or camouflage discrimination.
2. Discussion of the Prior Art
A proximity fuse of that type is already known from the disclosure of German patent specification 42 24 292 C1 as an optronic fuse arrangement with discrimination between masked or camouflaged and oblique targets through the utilization of dual-gate time-of-flight pulse measuring methods. With regard to a masked target, this relates to a more or less complete covering of the target which is actually of interest such as somewhat by means of shrubbery. Due to scatter phenomena which are encountered during the reflection of the emitted laser pulse, the received echo pulse is then significantly lengthier than the transmitting pulse which is reflected at an ideal target, at a more intensely sloped ascending and a much more intensely sloped descending flank of the course of the echo pulse which is received over the period of time. A similar pulse extension is encountered at stepped targets, such as in the instance of armored vehicles and their superstructures. For a corresponding discrimination, it is known from the prior publication of this type of class, to install in the projectile fuse two mutually orthogonally oriented, each independently operating laser radar devices of presently a transmitting and a receiving station. The target discrimination is effected through a prescription of threshold values for the correlation integral from the two echo signal configurations, which necessitates a considerable additional demand an equipment to be able to distinguish the two echo signal configurations from each other and for their correlative processing.
The optical radar which is known from the disclosure of German Patent Publication DE 39 15 627 A1 avoids a mechanical oscillation of the transmitting device, in that for the line-by-line scanning of the scene there are electronically sequentially interrogated a series of receiving elements, which are beam-geometrically associated with specified transmitting elements. Pursuant to the pulse time-of-flight method, from the individual echo pulse configuration which are received in a time multiplexer, there are then produced the range or distance images which are associated with the usual reflection images. Inasmuch as the receiving echoes, in effect; the received echo pulse configuration will intensely vary due to the different ranges to the target and an object reflectivites, in the signal detector at the receiving side thereof, for the initiating of the stop or range or distance trigger, there is implemented the so-called Constant-Fraction-Trigger-Principle.
In the laser range finding device which is known from the disclosure of German Patent Publication DE 196 07 345 A1, the transmitting pulse is spatially oscillated in such a manner as to also be able to pick up a reference reflector, whose range is known extremely precisely, so that for the actual time-of-flight range finding measurement there can be obtained corrective information therefrom. Thereby in addition to the start of the echo pulse configuration, obtained is also the charge which is received with the echo pulse, or instead thereof the length of the echo pulse configuration or shape. In order to determine the pulse length there is ascertained when a reference potential which is exceeded by the steeply ascending flank is contrastingly dropped below thereof by the much flatter descending flank. The result of the measurement in any event is more representative for the reflective conditions than for the reflector range or distance, inasmuch as the slopes of the ascending flank and of the rear flank of the echo pulse configuration are more representative of the pulse amplitude, and consequently besides the range, are above all extremely dependent upon the reflective properties of the reflector. This erroneous influence is computationally eliminated by means of the reference echo from an exact previously known range, which is naturally not realizable in a laser range fuse of a projectile approaching in flight to a potential target object due to the non-constant range and aspect-dependently varying reflective conditions.
In actual practice, as a result it has heretofore been considered to be adequate in a laser proximity fuse for target range finding measurement with the measuring of the pulse time of flight up to the point-in-time at which the rising or ascending flank of the echo pulse shape or configuration exceeds a reference potential which has been specified by the circuitry. The measurement point-in-time resultingly depends not only upon the actual range or distance to the reflected target, but also upon the momentary reflective conditions from which there are significantly influenced the echo amplitude, in accordance with to which there orients itself the steepness of the echo pulse configuration or shape. The criterium which is dependent upon the ascending flank as a result does not assure an optimum point in time for the triggering of the fuse. Moreover, disadvantageous for the triggering of the fuse at the beginning of the echo pulse configuration is that for masked targets or stepped echo structures, the triggering of the fuse is not optimized against the actual target center, but against a reflective condition located in front of the actual target center, such as in the case of shrubbery in front of an armored vehicle, so that no optimized attack on a target can be expected.
In recognition of these conditions, the present invention is directed to solving the technological problem in that from the flight of the projectile there can be derived an optimization of the triggering of the fuse even against stepped or masked targets with the aid of a laser range finder or measurement device.
In accordance with a combination of the primary features of the invention, the foregoing object is achieved in that for the initiation of the fuse is no longer triggered at the beginning rather but at the end of the echo pulse configuration, even though its comparatively flat descending or falling pulse flank is or is not adapted for the determination of a clear, reproducible or controllable triggering point-in-time. By means of the per se known Constant-Fraction-Trigger-Principle, which has heretofore been applied primarily to the rising or ascending flank of a pulse, in a surprising manner, there can be also derived a good controllable range or distance dependent triggering pulse for the thereby optimized initiation of the fuse from the flatter rear flank of the echo pulse configuration, whereby the attacking of the target will not take place prematurely, but will be better directed toward the target center.
An additional optimization in the initiation of the fuse can be inventively achieved when, in combination with the triggering based on the rear flank of the pulse, there are also considered the actually applicable target reflective characteristics, for which there is characteristic an elongation of the received echo pulse in comparison with the transmitted laser pulse, somewhat in view of the varying receiving conditions during approaching flight against a target which is either fully masked by fogging or shrouding, or a target which is partially masked by a local camouflage. For these additional optimization possibilities of the fuse initiating algorithm taking place upon triggering, which itself as such is not the subject of the present invention, pursuant to a modification of the invention, there is measured the pulse length ahead of the triggering pulse which is derived from the rear pulse flank. In order to thereby be able to eliminate the influences of the momentary echo amplitude and thereby the flank steepnesses as variable error sources which are dependent upon the approach to the target, the Constant-Fraction-Trigger-Principle is thus applied to both flanks of the echo pulse configuration or time-of-flight. This is particularly advantageous from the standpoint of apparatus by means of the provision of two comparators which are interconnected at the output sides with an AND-gate, and which are actuated through a single delay element for the received echo pulse configuration. For example, the delay element is connected to the direct (meaning, to the non-inverting) input of the one comparator and the inverting input of the other comparator, at an attenuated switching-in of the undealyed echo pulse configuration to the inverting input of the first mentioned comparator and the unattenuated activation of the direct input of the other comparator. As a result thereof, at the comparator outputs for the ascending flank, or respectively for the descending flank of the echo pulse configuration, there is always produced a bipolar differential. These two voltage sequences switch on the subsequent AND-gate only then and for so long, as the two sequences concurrently carry the logical switching level of the gate (for example, both positive signal potential), so that at the gate output there is generated a rectangular pulse of a length which is characteristics for the echo pulse configuration or shape, since these are now most extensively independent of the momentary peak value of the echo pulse configuration.